1. Field of the Invention
The present invention relates to a technique of generating light timing pulses from light signals, and particularly relates to a method of generating light timing pulses from a light signal with the attribute of the signal unchanged and a circuit for implementing such method.
2. Description of the Related Art
Research attempts have been made vigorously concerning communication methods based on optical time division multiplexing (optical TDM) as future optical communicating means which could provide very high transmission rates ("Optoelectronics; Device and Technology" journal, vol. 10, No. 4, pp. 447-460, December 1995).
Because optical time division multiplexing communication methods (optical TDM methods) perform data coding according to whether or not short pulse light exists (intensity modulation), they basically use return-to-zero (RZ) type light signals. However, present mainstream optical communicating methods use non-return-to-zero (NRZ) light signals. Thus, it may be necessary for future optical time division multiplexing communication methods to have a circuit for converting currently prevailing NRZ type optical data signals into RZ type ones. In addition, a circuit for generating light timing pulses in synchronization with the NRZ type optical data signal is required to implement an optical code conversion circuit. As one of the conventional light timing pulses extracting circuits, for example, such a circuit structure is known that has been disclosed in Japanese Published Unexamined Patent Application No. Hei 5-37469. In this circuit, an incident optical signal of NRZ type is input to an optical being amplified, is and after amplified, input to an intensity modulation to frequency modulation converting circuit (IM-FM converting circuit). By means of a nonlinear optical effect, the IM-FM converting circuit converts the intensity modulated light of NRZ type optical signal into frequency modulated light with an incident ray from a semiconductor laser. By extracting this frequency modulated light through a narrow-band-pass optical filter, light timing pulses are produced.
But, the above light timing pulses extracting circuit has such a bottleneck as follows. Using the nonlinear optical effect such as optical Kerr effect, the above-mentioned IM-FM converting circuit converts the variation in the intensity of NRZ-type optical data signals into the variation in frequencies of semiconductor laser rays. In order to use such nonlinear optical effect, it is necessary to amplify an incident NRZ-type light signal up to a high power level beyond several tens of milliwats (mW) before inputting it to the IM-FM converting circuit. Thus, the optical amplifier is required that it amplifies the light signal to an extremely large extent. This causes a problem that the power consumed by the entire circuit (particularly, the power consumed by the optical amplifier) is very large. There is another problem which stable operation of the circuit is not always expected because the nonlinear optical effect itself is a very unstable phenomenon. That is, the above-mentioned light timing pulses extracting circuit is questionable in its reliability, economical feasibility and operational stability.